A roller bearing is a cylindrical unit that is used to provide low-friction movement for a bushing or bearing block. A ball bearing is a spherical unit that accomplishes the same objective as a roller bearing. The real difference has to do with the contact surface between the bearing and the rail. For ball bearings (assuming a perfectly spherical bearing and no deformation), the contact surface is just a single point. Even once deformation is accounted for, the amount of surface where the ball is contacting the rail is limited. This creates an inherent strength limit for the balls. Roller bearings on the other hand, have an entire line of contact. This greatly increases the rigidity, stability, and maximum load capacity of the system.